H II region

An H II region is a volume of ionized hydrogen surrounding hot, young stars. Their ultraviolet radiation removes electrons from hydrogen atoms, and recombination produces characteristic emission.

Photoionization: The removal of electrons from atoms or molecules by absorbed photons. Stellar ultraviolet photons ionize hydrogen throughout the region.
H II: The spectroscopic notation for ionized hydrogen: a proton with its electron removed. It names the ionized hydrogen that defines the region.
Spectroscopy: The study of matter through its interaction with electromagnetic radiation across wavelengths. Line ratios and profiles diagnose the region’s temperature, density, and composition.
Orion Nebula: A nearby, bright nebula in Orion containing an active region of massive star formation. Its central Trapezium stars illuminate a well-studied H II region.
Strömgren sphere: An idealized, spherical ionized region whose size balances stellar ionizations against hydrogen recombinations. It estimates the boundary around a steady ionizing star in uniform gas.
H I region: A region dominated by neutral atomic hydrogen gas. Neutral hydrogen surrounds or lies ahead of the ionized gas.
Nebular emission line: A spectral feature produced by emission from diffuse ionized gas. H II regions are among the principal sources of nebular lines in galaxies.
Eagle Nebula: A star-forming nebula in Serpens containing the pillars known as the Pillars of Creation. Its ionized gas and sculpted molecular cloud show feedback at close range.
Hydrogen recombination: The capture of a free electron by a proton, forming a hydrogen atom and often emitting photons. Recombination powers much of the characteristic line emission.
Stellar classification: A system for grouping stars by spectral features, temperature, and luminosity. Hot O- and B-type stars supply the ionizing radiation.

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